We are proposing that a Conference entitled: Regulatory Functions of Interferons be held under the sponsorship of the New York Academy of Sciences. Interferons are known to exert a multitude of biological effects. Although the antiviral actions have been most thoroughly studied, the modulation of cellular susceptibility to virus infection is not the only important function of interferons. The idea that interferons act as immunoregulatory substances has now gained broad acceptance. The known effects of interferons on immune functions include the following: modulation of antibody response, inhibition of the mitogenic response of lymphocytes, activation of macrophages, enhancement of killer T cell activity and enhancement of the activity of a special population of lymphocytes termed "natural killer" (NK) cells. Two distinct types of "conventional" interferons are made by various cell types. In addition, distinct interferon activity is produced by sensitized T cells in response to specific antigen ("immune" or "type II" interferon) and it appears that this type of interferon (a lymphokine) is a more active immunoregulator than conventional interferons. Other regulatory effects of interferons include: effect on expression cell surface antigens, inhibitory action on cell division and tumor growth, inhibition of intracellular microorganisms (eg., chlamydia, rickettsia, protozoa). The potential of interferons as anti-tumor agents is now being evaluated in controlled clinical trials sponsored by N.C.I. and the American Cancer Society. Progress has recently been made in the elucidation of interferon action at the molecular level; these studies unexpectedly revealed the existence of molecular mechanisms of possible broad regulatory significance. The role of the proposed Conference would be to provide a coherent picture of the different regulatory functions of various interferons. The stress would be on interdisciplinary aspects that would be of interest to a broad audience of immunologists, cell biologists, molecular biologists and virologists.